Technical Field
The present invention relates to a method for injecting electrical energy and a device, in particular a wind power installation, for injecting the electrical energy.
Description of the Related Art
Nowadays, electric grids, which may be hereinafter also simply referred to as electric nets or electric networks, are increasingly supplied by regenerative sources of energy, such as wind power installations or wind farms exhibiting a different electrical behavior than common large-scale power plants, which employ at least one large-scale generator for injecting current. This means that such large-scale generators are being increasingly replaced with other injection units, such as cyclo-inverters. This is also referred to by experts as substitution. Countries like Germany, in particular, have a comparatively high degree of substitution, which means that comparatively many generators are replaced with other injection units. This may also have fundamental effects on the grid. The main concerns are that the possible balancing effects of the former injecting generators might be lost or at least weakened with an increasing degree of substitution.
This is why the proposed European Network Directive ENTSO-E provides for network operators to be able to demand an asymmetrical current injection. Here, the notion of symmetry or asymmetry relates to the correlation of the three phases of a three-phase grid to each other. Especially in the case of an asymmetrical disturbance in the grid, for example a short circuit between two phases or a short circuit of one phase to ground, it is provided to inject the electrical energy in as compensatory a manner as possible. A disturbance is to be assumed, in particular, if the actual voltage in the grid of at least one phase departs by more than 10% from its set point value and/or its rated value.
Initial objectives do exist, but they may not be reaching far enough.
The German Patent and Trademark Office has researched the following prior art in the priority application: DE 10 2006 054 870 A1; U.S. Pat. No. 7,423,412 B2; ANDERSSON, G.: Elektrische Energiesysteme—Vorlesungsteil Energieübertragung, p. 127-147, EEH—Power Systems Laboratory, ETH Zürich, September 2009; Symmetrische Komponenten, in Wikipedia, Die freie Enzyklopädie (Wikipedia, the free encyclopedia), Version of 23 Apr. 2012, URL: http://de.wikipedia.org/w/index.php?title=Symmetrische_Komponenten&oldid=102361863 [called up on 29 Jul. 2012].